where can i get a backdrop like this?
I would create a material on a plane with a spherical gradient. 



The method to tweak this gradient is changing the colorramp or the RGB-curve.  As you can see, the Fac slider of the RGB-curve is on 2.5. I used it to overexpose the white. 

Maybe it's better to adjust the color in the last step but that you have to test by your self ;-)